1. Field of the Invention
The present invention relates to a radar receiver and, more particularly, relates to a radar receiver of the type which is utilized in connection with a frequency-modulated, continuous-signal radar apparatus.
2. Discussion of the Prior Art
A radar receiver of the type which is considered herein is, in general, known from the disclosure of German Laid-Open Patent Appln. 31 13 472.
Furthermore, from the standard reference publication "Introduction to Radar Systems" by M. I. Skolnik, (Chapter 1.2), it is known that the power of a receiving signal which is returned upon being reflected from a pin-point target is inversely proportional to the fourth-power of the distance between the transmitting-receiving antenna and the pin-point target. Resulting therefrom is an extremely extensive dynamic range of the receiving signal in dependence upon variations in the distance of the radar relative to the pin-point target. This dynamic range cannot as a rule be controlled with a single demodulating-rectifier, inasmuch as either low input signals are at a level which is below the threshold of the characteristic curves, or high input signals will modulate the rectifier into the range of emission saturation, in essence, in both instances the fluctuations in the input will not lead to a representative output signal which would be suitable for digitalizing so as to enable a subsequent signal analysis. The conceivable alternative to respectively employing a rectifier or demodulator for low to average and for average to high excitations connected in parallel, is extremely complex from the standpoint of the circuitry technology, and is especially critical in the matching of characteristic curves during transition from low to high levels of excitations.
It is possible to conceive reducing the receiving signal dynamics through the installation of a compandor in the receiving channel; in effect, generally such as through the utilization of a logarithmic amplifier; for example, as is disclosed in U. Tietze and Ch. Schenk "Halbleiter-Schaltungstechnik" 5th Edition, pages 206 and 207, or through the effectuation of a characteristics curve for a compandor by means of a diode-series circuit. A frequency-dependently operating logarithmic amplifier; however, is extremely complicated from the circuitry technology. The implementation of a compandor characteristics curve from actual diode characteristic curves leads to design problems because of the non-linear (modulating) effects, with the requirement, that the higher harmonics resulting therefrom, must prior to the further processing, again be separately filtered out. Above all; however, it has been evidenced that the installation of such compandor circuits will deteriorate the antenna properties of the radar antenna inasmuch as the attenuation of secondary peaks becomes considerably lessened and, moreover, will adversely influence the unidirectional action of the radar antenna, inasmuch as the width of the main lobe is quite intensively increased; both of which is undesirable in the interest of obtaining a clear detection of a pin-point target in a clutter environment.